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• Using renewable energy generated electricity to drive the electrocatalytic reduction of CO₂ into valuable chemicals is an effective way to reduce the greenhouse effect and solve the energy crisis [1]. The electrochemical CO₂ reduction reaction (CO₂RR) involves multiple steps of electron and proton transfer, so multiple products can be produced [2]. Among the various products of the electrochemical CO₂RR, CO is favored because it is an important ingredient in many chemical reactions [3]. Hydrogen evolution reaction (HER) is a strong competitive reaction of CO₂RR [4], especially under the relatively negative potential. At a more negative potential, due to the limitation of CO₂ mass transfer [5], hydrogen (H₂) is more easily generated, so highly selective electrochemical CO₂ reduction can only take place in a narrow potential window. Therefore, it is of great significance to develop a catalyst that can inhibit hydrogen (H₂) evolution at a more negative potential for realizing the electrochemical reduction of CO₂ to CO under a wide potential window.

  Transition metal Cd-based catalysts have been widely studied due to their low cost and good performance in inhibiting H₂ evolution [6,7]. CdCO₃ in the Cd-based catalyst had the ability to activate CO₂ because it contained C atoms, which could combine with O in CO₂ [7]. Nevertheless, the activity of CdCO₃ for CO₂ reduction to CO was not satisfactory, which could only be greater than 90% under a narrow potential window (< 300 mV). Experimental and theoretical studies showed that the construction of heterostructure was one of the key strategies to improve the selectivity of CO₂RR products [8]. Heterostructure interfaces with abundant active sites accelerated the electron transport and regulated the adsorption barrier of intermediates [9,10]. For instance, the recent research indicated CdCO₃/Cd-CP heterostructure interfaces reduced the energy barrier of the *COOH intermediate, thus realizing the highly selective reduction of CO₂ to CO under a 500 mV wide potential window [10]. Although many efforts have been made, the highly selective (> 90%) electrochemical reduction of CO₂ to CO over CdCO₃ catalysts at a potential window was greater than 500 mV has not been reported.

  Recently, the microenvironment near the electrode has been proved to have a great influence on the selectivity of CO₂RR [11]. Raising the local pH near the electrode was of great benefit to inhibit HER, thus improving the selectivity of CO₂RR [12,13]. Luo et al. suppressed the formation of H₂ by using the OH⁻ formed in-situ preparation of rhombohedral CdCO₃ crystal (i-CdCO₃), and realized the highly selective reduction of CO₂ to CO under 500 mV wide potential window [13]. However, the high current density of CO₂RR would lead to the OH⁻ concentration near the electrode being many orders of magnitude higher than the bulk solution [14,15]. Excessive concentration of OH⁻ would react with CO₂ to generate CO₃²⁻, which was not conducive to inhibition of HER [16]. Therefore, adjusting the concentration of OH⁻ near the electrode to maintain an appropriate local pH value was particularly important for inhibiting HER and further improved the selectivity of the catalyst for CO₂ reduction.

  Here, we have enhanced the potential window of highly selective reduction of CO₂ to CO by combining the construction of heterostructure with the regulation of local pH. By adjusting the feed ratio of Cu²⁺ and Cd²⁺, the ability of the electrode to produce OH⁻ and adsorb OH⁻ could be adjusted. When the ratio of Cu²⁺/Cd²⁺ metal ions was 6:4, the catalyst exhibited the FE_co exceeded 90% under the wide potential window of 700 mV and the highest FE_co of 97.9% at −0.90 V (vs. RHE). Moreover, Cu_0.6/CdCO₃ showed excellent hydrogen evolution inhibition performance. The experimental results showed that an appropriate high local pH near the electrode inhibited the HER, and Cu_x/CdCO₃ composite structure created more active sites for electrochemical reduction of CO₂ to CO. The density functional theory (DFT) calculations also revealed that the excellent reduction of CO₂ to CO came from the adjustment of the local electronic structure of adjacent metal active sites by the Cu_x/CdCO₃ composite structure, which reduced the formation barrier of *CO intermediates.

  The synthesis of the catalyst was shown in Fig. 1a. Firstly, Cu_x/Cd(OH)₂ was prepared directly on carbon paper by using a one-step co-deposition method [17], where x represented the molar mass of Cu as a percentage of the total metal ion. Next, the Cu_x/CdCO₃ working electrode was prepared by cyclic voltammetry (CV) conversion in CO₂-saturated 0.1 mol/L KHCO₃. The microstructure of Cu_0.6/Cd(OH)₂ and Cu_0.6/CdCO₃ could be obtained from scanning electron microscope (SEM) and transmission electron microscopy (TEM) measurements (Figs. 1b-d). It could be seen that after the CV treatment, the morphology changed from nanosheets to irregular particles. X-ray diffraction (XRD) directly proved the transition from Cu_0.6/Cd(OH)₂ to Cu_0.6/CdCO₃, indicating the successful preparation of the composite catalyst (Fig. 1e). High-resolution TEM images (Fig. 1f) showed interfacial distances of 0.293 and 0.203 nm, which were consistent with (104) plane of CdCO₃ [18] and (111) plane of Cu [19], respectively. The element mapping image also showed that Cu, Cd, O, and C species were evenly distributed on the carbon paper (Fig. 1g). The measurement of lattice fringes and the uniform distribution of elements further indicated the successful preparation of a composite catalyst.

  Figure 1

  

  Figure 1.  (a) Schematic diagram of preparing Cu_x/CdCO₃ electrocatalysts. SEM image of the (b) Cu_0.6/Cd(OH)₂ and (c) Cu_0.6/CdCO₃. (d) TEM image of the Cu_0.6/CdCO₃. (e) XRD patterns of the Cu_0.6/Cd(OH)₂ and Cu_0.6/CdCO₃. (f) HRTEM image and (g) EDS-mapping images of the Cu_0.6/CdCO₃.

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  Fourier transform infrared (FT-IR) spectra of the samples before and after CV treatment was used to further demonstrate the successful carbonate transition (Fig. 2a). The disappearance of OH stretching vibration peak and the appearance of anti-symmetric stretching vibration peak and off-plane bending vibration absorption peak of CO₃²⁻ further proved the successful preparation of Cu_0.6/CdCO₃ [20,21]. The valence states of Cu and Cd on Cu_x/CdCO₃ surfaces were characterized by X-ray photoelectron spectroscopy (XPS). Compared with Cu_0.6/CdCO₃, the double peaks of Cd 3d in the CdCO₃ had a larger binding energy shift of 0.5 eV (Fig. 2b). On the contrary, the Cu 2p binding energy in Cu shifted to a smaller direction than that in Cu_0.6/CdCO₃ (Fig. 2c) [22]. XPS results demonstrated that there was a strong interaction between the electronic structures of Cu and Cd [23]. In addition, CO₂ adsorption capacity (Fig. 2d) tests were used to analyze the structural properties of the materials. The adsorption capacity of Cu_0.6/CdCO₃ for CO₂ was 2.45 cm³/g STP and 1.70 times that of CdCO₃ (1.44 cm³/g STP) (Fig. 2d). Stronger CO₂ adsorption capacity was conducive to subsequent CO₂ reduction [24].

  Figure 2

  

  Figure 2.  (a) FT-IR spectra of the Cd(OH)₂, CdCO₃, Cu_0.6/Cd(OH)₂ and Cu_0.6/CdCO₃. (b) Cd 3d XPS spectra of CdCO₃ and Cu_0.6/CdCO₃. (c) Cu 2p XPS spectra of Cu and Cu_0.6/CdCO₃. (d) CO₂ adsorption curves of Cu_0.6/CdCO₃ and CdCO₃ at 273.15 K.

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  Fig. 3a showed the linear sweep voltammetry curves (LSV) of Cu_0.6/CdCO₃, CdCO₃, and Cu in Ar and CO₂-saturated 0.1 mol/L KHCO₃ solutions, respectively. The current density in CO₂-saturated 0.1 mol/L KHCO₃ solutions was higher than that in Ar-saturated 0.1 mol/L KHCO₃ solutions, which preliminarily showed that the catalyst could electrochemically reduce of CO₂. According to researches, Cu was a post transition metal with low resistance and good conductivity [25]. CdCO₃, which belonged to the metal carbonate class, had a large resistance and a slow electron transfer rate compared to metals [26]. Therefore, at the same potential, CdCO₃ had the lowest current density in CO₂-saturated 0.1 mol/L KHCO₃ solution, while Cu exhibited the highest current density. The current density of catalysts with different proportions gradually increased with the increase of Cu content (Fig. S5 in Supporting information). The performance of the catalyst for CO₂ reduction under different applied potentials was tested by constant potential method. The Faraday efficiency (FE) of CO for Cu_0.6/CdCO₃ exceeded 90% in potential windows ranging from −0.70 V to −1.40 V (vs. RHE) and reached a maximum value of 97.9% at −0.90 V (vs. RHE) (Fig. 3b). By testing the CO selectivity of other samples with different proportions, it was found that the CO selectivity first increased and then decreased with the increase of Cu content, and reached the highest value when the Cu content was 60% (Fig. S7 in Supporting information).

  Figure 3

  

  Figure 3.  (a) LSV of the Cu, CdCO₃ and Cu_0.6/CdCO₃. (b) FE_co of the CdCO₃, Cu and Cu_0.6/CdCO_3. (c) Comparison of CO fractional current densities of CdCO₃, Cu and Cu_0.6/CdCO₃. (d) Stability test of Cu_0.6/CdCO₃ at −1.20 V vs. RHE.

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  In addition to FE, current density, energy conversion efficiency, and stability were also important indicators to evaluate the catalytic performance of CO₂RR. Cu_0.6/CdCO₃ had a superior CO component current density, which could reach a large value of 12.98 mA/cm² at −1.50 V (vs. RHE) potential (Fig. 3c). This was far more than the CO component current density of CdCO₃ (2.98 mA/cm²) and Cu (1.21 mA/cm²), indicating that it had an excellent catalytic activity of CO₂ reduction to CO. Moreover, according to the current density and potential function diagram generated by CO (Fig. 3c), when Cu_0.6/CdCO₃ generated CO, the overpotential required had a positive displacement of about 400 mV compared with that required by CdCO₃. These results showed that the composite structures could both increase the current density and reduce the overpotential of electrocatalytic CO₂ reduction to produce CO. Furthermore, Cu_0.6/CdCO₃ had outstanding cathode energy efficiency, which could reach 67.8% at −0.70 V (Fig. S8b in Supporting information).

  Long-term experiments were used to evaluate its stability. The electrodes were tested at a potential of −1.20 V vs. RHE (8.56 mA/cm²) for up to 12 h. As shown in Fig. 3d, the current density dropped slightly during the 12-h test, but the Faraday efficiency was still greater than 90%. The SEM (Fig. S13 in Supporting information), XRD (Fig. S14 in Supporting information), and XPS (Fig. S15 in Supporting information) of the samples did not change significantly before and after the long-term stability test. Therefore, Cu_0.6/CdCO₃ was a promising catalyst with excellent activity, high CO selectivity, and long-term stability.

  To better understand the source of the enhanced catalyst activity, the electrochemically active surface area (ECSA) was analyzed by testing the double-layer capacitance (C_dl) [27]. The C_dl value of Cu_0.6/CdCO₃ was 9.23 mF/cm² (Fig. 4a), 18 and 1.56 times higher than that of CdCO₃ (0.51 mF/cm²), and Cu (5.90 mF/cm²), respectively, indicating its abundant active sites on the surface. This result was consistent with the activity of CO₂RR. Furthermore, an electrochemical impedance spectroscopy (EIS) test was performed at the potential of −0.90 V (vs. RHE) to analyze the electron transfer resistance between the catalyst and the electrolyte [28]. Nyquist diagram showed that the charge transfer resistance and material transfer resistance of Cu_0.6/CdCO₃ were much smaller than that of CdCO₃ (Fig. 4b). Less resistance was beneficial to the rapid transfer of electrons and the substances [29,30], which could further promote the formation of intermediates in CO₂RR. The Tafel curve measurement (Fig. S10 in Supporting information) was used to study the dynamics of the CO₂RR process. Compared to Cu (181 mV/dec), CdCO₃ (259 mV/dec), and other ratios of Cu_x/CdCO₃ catalysts, Cu_0.6/CdCO₃ had the smallest Tafel slope (134 mV/dec). The Tafel slope of Cu_0.6/CdCO₃ was close to 118 mV/dec, indicating that the first electron transfer to CO₂ to generate CO₂⁻ was a rate-determining step (RDS) [31]. The minimum Tafel slope also meant faster CO formation kinetics [32].

  Figure 4

  

  Figure 4.  (a) Current density differences (Δj/2) plotted against scan rates of the Cu, CdCO₃ and Cu_0.6/CdCO₃. (b) Nyquist plots. (c) Oxidative LSV curves in Ar-saturated 0.1 mol/L KOH electrolyte of Cu_x/CdCO₃. (d) pH-dependence experiment on Cu_0.6/CdCO₃ (solid line) and CdCO₃ (dotted line) in 0.05 mol/L K₂HPO₄, 0.1 mol/L KHCO₃, 0.1 mol/L KCl solution at 2.71 mA/cm². (e) In-situ Raman diagram of Cu_0.6/CdCO₃ at different potentials. (f) Free energy diagrams of each intermediate during the electrochemical reduction of CO₂ to CO on the Cu_x/CdCO₃ and CdCO₃.

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  A rotating disk electrode was used to directly measured the OH⁻ generated near the electrode [33]. As the applied potential on the disk increases, CO oxidation peak (E_obs) moved in a more negative direction (Fig. S12 in Supporting information), which directly proved the generation of OH⁻ on the electrode. According to the LSV of samples with different proportions, the current density of the sample increased with the increase of Cu content, which meant that the ability to produce OH⁻ was also proportional to the Cu content. At the same time, the oxidation test curve in Ar-saturated 0.1 mol/L KOH solution was used to test the overpotential required for OH⁻ adsorption (Fig. 4c). The results showed that the overpotential of OH⁻ adsorption decreased with the increase of Cu content, which meant that the introduction of Cu could enhance the adsorption of OH⁻ [34]. Further comparing the selectivity of different catalysts for H₂, it was found that Cu_0.6/CdCO₃ had the lowest selectivity for H₂ over the entire tested potential range (Fig. S7 in Supporting information). Therefore, the pH value of Cu_0.6/CdCO₃ surface was optimal, which was more conducive to inhibition of HER. To further study the effect of local pH on CO₂RR, the CO₂RR of Cu_0.6/CdCO₃ and CdCO₃ were tested at a constant current density of 2.71 mA/cm² at 0.05 mol/L K₂HPO₄, 0.1 mol/L KHCO₃ and 0.1 mol/L KCl, respectively [12]. The buffering capacity of K₂HPO₄, KHCO₃, and KCl decreased in turn, while the local pH value increased in turn. As shown in Fig. 4d, with the increase of local pH, the FEh₂ became lower, and the FE_co became higher. This further proved that moderately high local pH was favorable for CO₂RR and unfavorable for HER.

  In-situ Raman spectroscopy (Fig. 4e) was used to test the reaction path of electrochemical reduction of CO₂ to CO by Cu_0.6/CdCO₃. The peak at 250 cm⁻¹ was attributed to the Cu-CO stretching mode [35], which became stronger with the increase of potential. The peaks at 523 and 1030 cm⁻¹ were attributed to Cu_xO [35] and *OCO intermediate [36]. After protonation, the *OCO intermediate was transformed into *OCHO, which was conducive to the formation of HCOOH [36]. According to the in-situ Raman results, the peak of *OCO was obvious at high potential (< −1.20 V vs. RHE), indicating that a small amount of HCOOH was generated at high potential, which was consistent with our experimental results. The Cu-OH peak at 706 cm⁻¹ appeared from the −0.70 V vs. RHE [37]. When the applied potential was less than −1.20 V vs. RHE, the peak became more obvious. This directly proved the existence of OH⁻ on the surface of the catalyst and further validated the local pH rise. The DFT calculation was performed to further analyze the active source of Cu_x/CdCO₃ for high selectivity CO₂RR. The calculated Gibbs free energy (ΔG) diagram of each intermediate was shown in Fig. 4f. On the surface of Cu_x/CdCO₃, the ΔG required for the transition from *COOH to *CO was 1.07 eV, which was much smaller than the 2.97 eV of CdCO₃. This indicated that Cu_x/CdCO₃ was more conducive to the formation of *CO [38]. The d-band center was the key to determining the electrochemical activity, which could be evaluated by partial state density (PDOS) data [39]. Compared with CdCO₃, the d-band center of Cd in Cu_x/CdCO₃-*COOH was closer to the Fermi level, while the d-band center of Cd in Cu_x/CdCO₃-*CO was further away from the Fermi level (Fig. S19 in Supporting information). This proved that the Cu_x/CdCO₃ surface had a strong adsorption energy for *COOH and a weak adsorption energy for *CO [40]. These results showed that the strong electron interaction between Cu and CdCO₃ could reduce the formation energy barrier of *CO intermediate and promote the desorption of CO from Cu_x/CdCO₃ surface, so that the electrochemical CO₂ reduction to CO had a fast reaction kinetics.

  In summary, Cu_x/CdCO₃ catalysts were prepared for the first time by simple electrodeposition and CV treatment. Cu_0.6/CdCO₃ exhibited excellent performance of reducing CO₂ to CO, high CO partial current density and fine stability. The experimental results proved that the excellent performance of Cu_0.6/CdCO₃ catalyst came from the reasonable adjustment of OH⁻ generation and adsorption, which inhibited HER. Theoretical calculations have proved that Cu_x/CdCO₃ catalyst reduced the energy barrier from *COOH to *CO intermediate, thus realizing the highly selective electrochemical reduction of CO₂ to CO under a wide potential window. This work explored the influence of local microenvironment and electronic structure on the CO₂RR, which will provide new guidance for the design of highly selective electrocatalysts under a wide potential window.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 22225808, 22075111), Sino−German Cooperation Group Project (No. GZ1579), Jiangsu Province Innovation Support Program International Science and Technology Cooperation Project (No. BZ2022045).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108520.

  
  
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• 

  Figure 1  (a) Schematic diagram of preparing Cu_x/CdCO₃ electrocatalysts. SEM image of the (b) Cu_0.6/Cd(OH)₂ and (c) Cu_0.6/CdCO₃. (d) TEM image of the Cu_0.6/CdCO₃. (e) XRD patterns of the Cu_0.6/Cd(OH)₂ and Cu_0.6/CdCO₃. (f) HRTEM image and (g) EDS-mapping images of the Cu_0.6/CdCO₃.

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  Figure 2  (a) FT-IR spectra of the Cd(OH)₂, CdCO₃, Cu_0.6/Cd(OH)₂ and Cu_0.6/CdCO₃. (b) Cd 3d XPS spectra of CdCO₃ and Cu_0.6/CdCO₃. (c) Cu 2p XPS spectra of Cu and Cu_0.6/CdCO₃. (d) CO₂ adsorption curves of Cu_0.6/CdCO₃ and CdCO₃ at 273.15 K.

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  Figure 3  (a) LSV of the Cu, CdCO₃ and Cu_0.6/CdCO₃. (b) FE_co of the CdCO₃, Cu and Cu_0.6/CdCO_3. (c) Comparison of CO fractional current densities of CdCO₃, Cu and Cu_0.6/CdCO₃. (d) Stability test of Cu_0.6/CdCO₃ at −1.20 V vs. RHE.

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  Figure 4  (a) Current density differences (Δj/2) plotted against scan rates of the Cu, CdCO₃ and Cu_0.6/CdCO₃. (b) Nyquist plots. (c) Oxidative LSV curves in Ar-saturated 0.1 mol/L KOH electrolyte of Cu_x/CdCO₃. (d) pH-dependence experiment on Cu_0.6/CdCO₃ (solid line) and CdCO₃ (dotted line) in 0.05 mol/L K₂HPO₄, 0.1 mol/L KHCO₃, 0.1 mol/L KCl solution at 2.71 mA/cm². (e) In-situ Raman diagram of Cu_0.6/CdCO₃ at different potentials. (f) Free energy diagrams of each intermediate during the electrochemical reduction of CO₂ to CO on the Cu_x/CdCO₃ and CdCO₃.

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